Media captionDavid Cameron: "This is the start of a journey, but there is also the option of introducing stronger laws here in the UK and I don’t rule that out"

The prime minister has warned internet companies that they need to act to block access to child abuse images or face new legal controls.

David Cameron told the BBC's Andrew Marr Show search firms like Google must do more to stop results from "depraved and disgusting" search terms.

Google said when it discovers child abuse images it acts to remove them.

Labour said the PM's plans did not go "far enough" and criticised cuts to online child abuse policing budgets.

Google is one of a number of firms which recently agreed on measures to step up the hunt for abusive images.

In June, after a meeting chaired by the culture secretary, the government said Google and others including Yahoo!, Microsoft, Twitter and Facebook would allow the charity the Internet Watch Foundation actively to seek out abusive images, rather than just acting upon reports they received.

'Big argument'

The prime minister said he wanted search companies to go even further and block certain search terms from providing results.

Analysis

The prime minister has now applied intense political pressure on Google - and other search companies - to do more to block access to child abuse images.

But civil liberties campaigners fear that blocking certain searches in one country could set a precedent elsewhere, making other governments more confident in applying censorship.

In any case, many child protection experts are dubious about the effectiveness of the policy - they say most illegal images are hidden on private forums, in cyber-lockers, and on peer-to-peer networks, and are not available via search engines.

For its part, the government says companies always raise technical objections to this kind of initiative and they need to use their technology to find solutions.

He predicted that the call would prompt a "big argument", but he warned: "If we don't get what we need we'll have to look at legislation."

The interview precedes a speech on Monday, in which Mr Cameron is expected to provide more details of the government's plans.

Anyone searching for a word on a "blacklist" compiled by the Child Exploitation and Online Protection Centre (Ceop) should be made to view a webpage warning them of the consequences, "such as losing their job, their family, even access to their children", Mr Cameron will say.

"There are some searches which are so abhorrent and where there can be no doubt whatsoever about the sick and malevolent intent of the searcher," the PM will add.

He will tell the internet companies: "If there are technical obstacles to acting on this, don't just stand by and say nothing can be done; use your great brains to help overcome them."

Why can't they take this stuff off the internet? Kids are getting killed, abused, raped and messed up for the rest of their lives. What's their excuse? I think it is money.Paul Jones, the father of murdered schoolgirl April

A spokesman from Google said: "We have a zero tolerance attitude to child sexual abuse imagery. Whenever we discover it, we respond quickly to remove and report it.

"We recently donated $5m (£3.3m) to help combat this problem and are committed to continuing the dialogue with the government on these issues."

Shadow home secretary Yvette Cooper said: "Child abuse is a hideous crime and its scale on the internet is deeply worrying.

"David Cameron said he would make sure the police had the resources. But the truth is that Theresa May has cut by 10% the resources for the Child Exploitation and Online Protection Agency," she said.

Despite identifying 50,000 cases of British residents accessing images of child abuse online last year, Ceop had pursued only about 2,000, she added.

'Very secretive'

The debate about online images showing the sexual abuse of children has come to prominence following two high-profile court cases in which offenders were known to have sought child pornography online.

Mark Bridger, sentenced to life in May for the murder of five-year-old April Jones in Machynlleth, Powys, searched for child abuse and rape images.

And police who searched the Croydon home of Stuart Hazell, jailed for life in May for murdering 12-year-old Tia Sharp, said they had found "extensive" pornography featuring young girls.

But Jim Killock, executive director of the Open Rights Group which campaigns for online freedoms, said: "The idea that banning some search terms will reduce the amount of child pornography online is a bit of a mistake.

"While I think David Cameron is very well intentioned, and of course everybody wants this kind of material to be tackled, we have no real evidence that search engines are the major way that people try to find this material.

"Because it's very, very illegal, people tend to be very secretive."

It would be better to boost funds for the policing of the criminal gangs and private networks responsible for the production and distribution of child abuse images, and to crack down on the methods used to pay for them, he said.

But John Carr, from the Children's Charities' Coalition on Internet Safety, said the PM was "absolutely right: there is more that can be done and should be done."

He conceded the plans would not hinder the "tiny, tiny proportion of highly technically literate paedophiles".

"But there's a whole group of others. I mean, we know about two of them, because they were caught and convicted in those murder trials. The judge pointed to the way that they had used the internet to feed their murderous interest and depraved sexual interest in children," he said.

"That kind of thing we can stop."

During a meeting with Mr Cameron at Downing Street, reported in the Sun newspaper, Paul Jones, the father of murdered schoolgirl April, said: "Why can't they take this stuff off the internet? Kids are getting killed, abused, raped and messed up for the rest of their lives.

"What's their excuse? I think it is money. They have the technology and they can do this."

Mr Cameron also said he would like to see more restrictions on access to legal pornography that can be seen online by children.

"There are rules about what films you can see in a cinema, what age you have to be to buy alcohol or cigarettes.

"But on the internet, there aren't those rules, so we need to help parents with control."